Cored wall construction



1967 1...]. HESSBURG, JR, ETAL 3,303,617

CORED WALL CONSTRUCT ION Filed April 16, 1963 //V[//V70/?5 THOMAS c. OLSON LAWRENCE J. HESSBURG,JR.

Arm/W575 United States Patent 3,303,617 CORED WALL CONSTRUCTION Lawrence J. Hessburg, Jr., St. Paul, and Thomas C. Olson, Minneapolis, Minn., assignors to Stainless & Steel Products Co., St. Paul, Minn., a corporation of Minnesota Filed Apr. 16, 1963, Ser. No. 273,348 17 Claims. (Cl. 52249) This invention relates to wall construction of the cored or sandwich type comprising two spaced apart skins, panels, sheets or outer walls having inner core material such as insulation disposed therebetween, and particularly to that type of wall construction in which the core material has inherent structural strength and is bonded to each of the outer walls to structurally unite them.

More particularly, this invention relates to cored wall construction in which the core material is susceptible to low temperature dimensional instability, that is, contraction or shrinkage.

When sandwich type wall construction utilizing temperature-sensitive core material is subjected to a significant drop in temperature, the shrinkage of said core material occasioned thereby oftentimes structurally weakens the cored wall construction, cause buckling, wrinkling and failure of one or both of the outer walls and breaking of any bond between the core and the outer walls and the formation of voids between the core and the walls. This problem is particularly acute when such cored wall construction is of annular shape or cross section such as in liquid transport tanks.

An important object of this invention is to provide a novel method of making cored wall construction embodying temperature-sensitive core material which Will not structurally fail when subjected to low temperatures, and which overcomes the aforementioned problems relating .to shrinkage.

Another object is to provide a novel cored wall construction embodying temperature-sensitive core material which is structurally superior to what has been available up to now and which is especially suited for use in annular structures such as transport tanks, and which is structurally stable under ambient temperature conditions if made in accordance with the method of this invention.

Another object is to provide a novel cored wall construction utilizing a plastic foam, preferably urethane foam, as the core material, and to provide a novel method of forming said wall construction so as to prevent failure thereof during use due to shrinkage of the core material.

These and other objects and advantages of this invention will more fully appear from the following description made a in connection with the accompanying drawings wherein like reference characters refer to the same or similar parts throughout the several views, and in which:

FIG. 1 is a side view partially in section and partially in elevation of a trailer t-ank constituting one preferred embodiment of the present invention;

FIG. 2 is a cross sectional view of FIG. 1 taken on the line 2--2 thereof;

FIG. 3 is a diagrammatic cross sectional view of a portion of a cored wall formed according to one method of this invention;

FIG. 4 is a diagrammatic cross sectional view of a portion of a cored wall formed according to another method of this invention; and

FIG. 5 is a diagrammatic cross sectional View of a portion of a cored wall formed according to still another method of this invention.

The method of this invention broadly and briefly stated comprises forming at least a portion of the core and reducing the volume of the formed portion prior to completing the assembly of the cored wall. This reduction in volume may be accomplished in several ways, including pre-shrinking and/ or compressing all or part of the core material, and i discussed in more detail hereinafter in considering the applicability of the method of this invention to the formation of the urethane foam tank construction of this invention.

One preferred embodiment of .this invention utilizes a double walled sandwich construction for a liquid transport tank such as for hauling milk and the like. In tank transports, the tare weight thereof is of critical importance, because of the maximum weight limitations imposed by law, and each pound saved in total tare weight means a pound more of payload which may be hauled. Thus, it is important that the manufacturers of trailer tanks make the total capacity as large as possible within the weight limitations imposed consistent with adequate strength in order that the carrier may achieve a maximum payload on each trip.

We have found that the use of a plastic foam, particularly urethane foam, as the core material provides a trailer tank superior to any previously devised, particularly when the tank is formed in accordance with the invention. By utilizing a plastic foam having inherent structural strength, and bonding or otherwise securing the foam to both the inner and outer walls, the inner and outer walls and the core material form an integral monolithic wall structure in which both the inner and outer walls and the core contribute to the support of the weight of the load carried by said tank, and contribute in resisting the stresses of static and dynamic loading. This arrangement enables lighter weight met-a1 to be used for the inner and outer tank walls Without sacrificing tank strength, whereby the ratio of the weight of the tank to its volume may be materially reduced, thereby increasing the volume or payload which may be carried for any given tare weight, and effecting a considerable saving in the cost of construction of said tank.

We have found that urethane foam is especially desirable as the core material because it has the lowest thermal conductivity of known plastic foams and because of its low density, structural strength, its ability to bond to metal such as stainless steel, and its ability to completely fill any cavity into which it is introduced. Structurally, the urethane foam sandwich construction is vastly superior to conventional non-bonded wall construction in which the core insulation is not bonded to the inner and outer shells.

FIGS. 1 and 2 illustrate a typical embodiment of the present invention. The jacketed insulated trailer tank assembly shown in the accompanying drawings includes an inner material receiving tank or shell 10 which may be of any cross sectional shape desired including cylindrical or oval, and is preferably of as thin walled construction as possible. The tank 10 is encircled at axially spaced intervals by reinforcing ring members 11 for circumferentially reinforcing the tank. Two elongate, laterally spaced apart, longitudinally extending, tank supporting frame members 12 are provided which extend the full length of the tank 10 and underlie and support the same by interconnecting the ring members. The frame members 12 are interconnected by means of transverse crossmembers or bolsters 13 which extend therebetween and are secured thereto as by welding.

Elongate upwardly tapered side outrigger members 14 are provided on each side of the reinforcing rings 11. The lower ends of the outriggers 14 are welded to the frame members 12. The outriggers 14 extend diagonally outwardly and upwardly from the supporting members 12 so as to underlie and receive a portion of their respective rings and are welded to the rings to integrally fasten the outriggers t0 the rings to distribute the stress from the running gear into the rings. Elongate compression members 16 may be provided on the upper half of the tank to reinforce same, the tank also being provided with a conventional manhole cover M providing access to the interior of the tank.

The tank is completely enclosed with a core 13 of insulating material having inherent structural strength, which core, according to the present invention, is preferably a plastic foam and more particularly urethane foam. The core in turn is enclosed by jacket means, the

side and top of the tank being enclosed by a jacket 19, the lower ends of which are fastened to the supporting frame members 12 to anchor the jacket portion 19 to the supporting structure. The underside of the tank is enclosed by an additional jacket member 19a to complete the enclosure of the tank and insulation, jacket 19a also being anchored to the supporting members 12. The core is joined to both the inner and outer Walls to structurally unite same and provide a monolithic wall structure. In the illustrated embodiment, the foamed core is selfbonded directly to the inner and outer walls. However, it will be understood that it is within the scope of this invention to structurally unite the core with the inner and outer walls by any suitable means which would also include any suitable adhesive means effective for that purpose.

The running gear indicated in the entirety by the letter R is mounted on and suspended from the frame members 12 by means of the bogie frame 20 which is secured directly to the underside of the frame members 12 in any suitable fashion as by welding. Other conventional appurtenances such as the landing legs L, the fifth wheel assembly W, miscellaneous cabinets (not shown), the fenders F and the ladders X are also fastened to and supported by the main supporting structure hereinbefore described.

Thus, to illustrate the advantages of a foamed doublewalled transport tank in accordance with this invention, such a tank permits a saving in the weight of metal used in the wall structure, which saving may range from-200 pounds in a small tank to 1000 pounds or more in a maximum capacity transport. In addition, it permits an increase in the maximum legal capacity by at least 100 gallons. In addition, this type of foamed double-walled tank construction effects a material reduction in labor costs, there is no moisture pick-up in the insulation, and there is a 50% reduction in the heat transfer coefficient.

To further. illustrate the strength of double-walled construction utilizing urethane foam core material bonded to both walls, tests were run on cored wall sections or panel's two inches by eight inches by twenty-six inches. The foam core was a typical three pound per cubic foot Freon blown polyether-urethane foam. The top skin was 20 gauge steel, the bottom skin 16 gauge steel. The foam was poured in such a fashion to cause it to rise in the eight inch direction. Fatigue testing on these panels indicated that given a good original bond, they are not subject to fatigue failure and are capable of withstanding 10,000,000 cycles at stresses approaching the yield point of the steel. The tests were run by simple supporting the panels on 20 inch centers with a concentrated alternating load applied at mid-span, the loading bar and the two supports making contact with the panel in the eight inch direction full width, with an alternating load applied thereto.

Flexure tests on the panels indicate that a three pound per cubic foot core density will stand a total quarter point load of 900 pounds. Other densities take loads that are correspondingly lower or higher. Also, a three pound per cubic foot core density with skins of 16 and 22 gauge stainless steel will stand five times the load of a non-bonded panel of Styrofoam utilizing 12 and 18 gauge stainless steel.

Lengthwise buckling tests performed on the panels showed that a three pound per cubic foot core density panel would take an average maximum load of more than 3,000 pounds. In contrast, a Styrofoam panel with 14 and 20 gauge stainless steel skins is capable of taking only 135 pounds. Thus, foamed urethane panels are 22 times more buckle resistant than corresponding Styrofoam panels.

Welded stress rings are one of the primary causes of stress concentration on the inner shell. The use of structural core material, particularly urethane foam, bonded to both the inner and outer walls, minimizes the effect of stress risers on the inner shell and permits the elimination of some or all thereof. Thus, this type of construction permits a reduction in the size and number of the stress rings, and in some instances permits the elimination of welded rings all together, such as in conventional cradle type construction. The foamed urethane construction also provides about a ten-fold multiplication of the buckling strength at top dead center of the tank and provides for an even and gradual distribution of stresses from the loading points (the bogie, the fifth wheel and landing gear, etc), with no trouble with pounding up of the bottom.

To illustrate the saving in weight and increase in payload which the urethane foam construction of this invention provides over conventional designs, the following comparison is provided. A transport tank of conventional design having a 5700 gallon capacity, Styrofoam insulating core material which is not bonded to either the inner tank or outer jacket, an inner stainless steel shell having a 10 gauge bottom, 12 gauge top and 14 gauge sides, with the inner shell constituting the sole load supporting medium and a 20 gauge jacket, weigh-s 10,913 pounds, complete with running gear, auxiliary equipment, etc. A corresponding transport tank made according to this invention utilizing an inner shell of 16 gauge stainless steel, a urethane foam core bonded to both inner tank and outer jacket and having a density of 2.6 pounds per cubic foot and a 20 gauge jacket has a capacity of 5800 gallons and weighs only 10,043 pounds. Thus, in the foregoing example, the urethane foam construction of this invention provides an increase of gallons in the payload or capacity of the conventional tank while at the same time reducing the total weight of the tank by some 870 pounds.

We have also invented a novel method of fabricating cord wall construction utilizing temperature-sensitive core material, the method being typified by its applicability to our aforedescribed urethane foam tank construction as set forth hereinafter, the urethane foamed tank of our invention also typifying the typical shrinkage problem associated with the use of temperature-sensitive core material, unless the wall structure is fabricated in accordance with the method of this invention, the method of this invention'being applicable to overcome the problem of shrinkage when urethane foam or any other temperaturesensitive core material is utilized in a sandwich wall construction.

A sandwich wall construction utilizing urethane foam as the core material would not be expected to present any problems insofar as shrinkage is concerned, since the published literature and the formulators of urethane foam give no indication that urethane foam is susceptible to material low temperature dimensional instability and, in fact, the formulators almost universally claim that their foam does not shrink to any significant extent.

However, when a cored wall such as the aforedescribed tank of our invention is formed by pouring the urethane foam between the two shells, whereby the foam reacts and cures so as to bond itself to both the inner and outer shell simultaneously, we have found that the foam radically contracts and shrinks during cure and when subjected to low temperautres after cure. Where the cored wall is an annulus such as a tank, the outer shell tries to follow the shrinkage of the foam and reduce its diameter. For example, a typical total shrinkage of 3% would be a shrinkage'of 0.09 inch in a three inch thickness,

resulting in a diameter reduction of 0.18 inch and a circumferential reduction of more than one-half inch. However, the outer shell refuses to yield uniformly to the change in shape caused by the shrinkage of the foam and this results in a localized bond failure between the foam and the outer shell, caused by localized buckles or wrinkles therein.

Once this bond failure is started, it propagates rapidly when stressed, because the foam bond has a relatively low peel strength. When the bond fails substantially, the structural integrity of the foam sandwich is ruined and its structural advantages are lost.

We have found that failue due to shrinkage of temperature-sensitive core material can be minimized or prevented by our novel method of fabricating cored wall constructions of this type, which method involves controlling either or both of the two most important factors involved in the failure of cored walls in general, and cored annuli such as these foamed core tanks in particular. One factor is the shrinkage of the core material during formation, especially curing thereof, the other factor being shrinkage of said core material due to a drop in temperature thereof after curing of the core material and completion of the integral monolithic wall structure. Broadly speaking, these factors are controlled and overcome according to this invention by reducing the volume of a given amount of core material after formation thereof and prior to completing the cored wall assembly. According to this invention, the bond failure attributable to the shrinkage of core material during curing thereof can be avoided or minimized by preshrinking all or part of the core material before it is bonded to both skins or walls.

The destructive shrinkage caused by low temperatures after curing can be overcome or at least minimized according to this invention by prestressing or compressing the cured or otherwise formed core material to compensate for later dimensional instability or shrinkage due to a drop in temperature. Thus, preshrinkage, prestressing or both may be employed according to this invention to avoid failure of the tank construction due to shrinkage of the core material, the particular relief means employed depending on the particular materials used as the core material, and the stresses and temperature changes which the finished product is likely to be subjected to during use.

In the case of urethane foam, it has been found preferable to employ both preshrinkage and prestressing, with the preshrinkage being considered to be the most important single factor because the cure shrinkage is usually greater than that encountered by a post-curing temperature drop. Thus, we have experienced 27% shrinkage of urethane foam during curing (3% being typical), and

.5-2.5% shrinkage over a temperature drop of approximately 80 F. (room temperature to F.). Urethane foam is a result of the polymerization of polyethers or polyesters and toluene diisocyanate (TDI) in the presence of a blowing agent such as trichloromonofluoromethane (Freon 11). The reaction of the component is exothermic and as such it causes the Freon (B.P. 74.8 F.) to vaporize. This vaporization is confined in that it occurs in small cells formed by encapsulation of liquid Freon during reactant mixing. The reaction expands the liquid 10 to 40 times depending on the Freon concentration and then solidifies it. As the foam cools, the Freon contracts and partially collapses the cells, resulting in the shrinkage during cure.

The preshrinkage of the foam may be accomplished in any one of several ways in accordance with this invention. Thus, for example, the entire core may be foamed, formed and permitted to shrink prior to bonding of either of the walls thereto, or the entire thickness of the foamed core material may be laid up on one of the walls and permitted to cure thereto and complete its shrinkage during curing while only attached to the single wall, after which the other wall may then be bonded to the core material.

For illustration of the foregoing, reference is made to the cored wall section diagrammatically depicted in FIG. 3. Thus, the core material 21 would be initially foamed to thickness a, either by spraying or pouring and either completely independently of either of the walls 10 or 19, or laid up against one of them, such as 10, and then permitted to cure, and shrink to thickness b, after which the bonding of the core to both walls 10 and 19 and the assembly of the monolithic cored wall would be completed.

Preshrinkage can also be accomplished according to this invention by forming and curing a part of the core on one or both of the outer skins or walls, followed by a filling of the remaining voids existing between the two walls by the rest of the core material to complete the integral connection between the two outside walls by means of the interior core material. Where foam such as urethane foam is the core material, the pressure exerted by the foaming of the core material in forming the remainder of the core as aforementioned, effects the desired prestressing of the already cured foam to compensate for subsequent shrinkage or contraction due to exposure to low temperatures after curing and formation of the sandwich.

The aforementioned techniques are diagramatically illustrated by the cored wall sections of FIGS. 4 and 5. In FIG. 4, core material such as spray foam 22 is initially laid up against and self bonded to jacket 19 to the initial thickness c and then permitted to cure and shrink to thickness d. The void represented by dimension e is then filled with poured foam 23 which bonds itself to spray foam 22 and tank 10 to form the desired monolithic wall. The poured foam expands and exerts a foaming pressure on the spray foam 22 and compresses and prestresses same whereby the spray foam after the poured foam is cured and the wall assembly is completed, has the final prestressed thickness 1 and the poured foam has a final thickness g, the difference h between the spray foam thicknesses d and 1 representing the amount of compression and prestressing thereof effected by the poured foam.

In FIG. 5, spray foam 24-24 is applied to both walls 10 and 19 and permitted to cure and shrink to their respective thicknesses i, the spray foam self bonding itself to its respective wall. The void 1 between the cured spray foam layers 2424' is then filled with poured foam 25 which bonds itself to the spray foam 24-24 to interconnect the jacket 19 and tank 10 and form a monolithic wall structure therewith. The poured foam expands and exerts a foaming pressure on and compresses and prestresses both spray foam layers 24 and 24', whereby the spray foam layers, after the poured foam is cured and the wall assembly is completed, have the final thicknesses k and the poured foam has the final thickness 1, the difference m between the spray foam thicknesses i and k representing the amount of compression and prestressing thereof effected by the poured foam.

Prestressing may also be accomplished by compressing the cured core material in any suitable fashion such as by pulling the outer shell or jacket tightly over the core material so as to compressively prestress the core material between the two shells or walls.

To illustrate same, reference is again made to FIG. 3, wherein the thickness b represents the preshrunk, cured thickness of the core material 21. The cured preshrunk core 21 is bonded to the tank 10 by any suitable means. The core is then compressed and prestressed to the thickness n and the jacket 19 is bonded thereto to maintain the compressed prestressed dimension of the core, which compression of the core and laying up of the jacket may be accomplished in any suitable fashion, such as by the aforementioned method of pulling the jacket down so tightly on the core as to effect the compression desired.

Where a foamed material is to be used for the core,

we have found it preferable to utilize the foam in the sprayed form rather than in the poured foam wherever possible. Spray foam is about twice as dimensionally stable as poured foam because the atomization thereof breaks up the cells into a finer more uniform and stronger network. The cells are spherical and not weakened by elongation normally encountered with poured foam.

Another reason for the preference for spray foam is that spray foam exhibits a better bond to stainless steel or any other metallic surface because it is blasted at the surface.

Where the fabrication technique embodies a combination of initially spraying part of the core or one wall only, followed by pouring of foam to fill the void between the sprayed foam and the other wall, it is best to spray the foam on the jacket or outer shell first. The bond between the core material and the outer jacket is the one which is subjected to the greater strain and stress during shrinkage. It is, therefore, preferable to employ that technique, namely spraying, which will accomplish the strongest bond between the foam and the jacket. Thus,'as in FIG. 4, the foam 22 is preferably sprayed on the jacket 19, and the foam 23 is poured to complete the formation of the core and wall assembly. Also, where a combination of sprayed and poured foam is to be used, it is desirable to have as much of the core sprayed as possible, leaving only enough space for poured foam which permits proper rising, foaming and curing of the poured foam.

As a typical example of the success of our method of fabricating cored wall construction utilizing temperaturesensitive material, an insulated double walled trailer tank was formed by spraying 2" of urethane foam on the inside of the jacket (20 gauge), with none being sprayed on the inner shell (16 gauge). The jacket was then installed over the inner shell and urethane foam was poured into the remaining cavity whereby the poured foam bonded itself to the inner shell and the sprayed foam, and the inner and outer shells became strongly bonded together, the total core thickness being 2 /2". When subjected to severe freeze tests to degree F. temperatures for protracted periods) this assembly showed no signs of structurally failing, and the outer walls showed no buckles or wrinkles thereon.

Thus, a preferred trailer tank of this invention embodies urethane foam as the core material, with the core being bonded to both the inner and outer walls whereby both walls and the core material are structurally integrally united and function as a monolithic structure to support the load carried by the tank. The aforementioned tank and any other tank made from core material which shrinks during formation of the assembly is, ac-

cording to this invention, first preshrunk prior to bonding of the core material to both of the walls of the sandwich and in some instances, it is preferable to include a prestressing of the preshrunk or finished or completely cured core material so as to compensate for subsequent exposure to low temperatures and resultant shrinkage after the assembly of the cored wall construction has been completed.

The sprayed foam hereinbefore referred to is formulated and mixed in the same manner as poured foam, but has a higher catalyst level than poured foam so that the reaction is faster so that the spray foam will not run off an inclined surface. Spray foam is formed by atomizing the mixed material with compressed air and spraying the atomized material on an exposed surface. Poured foam is formed by dispensing the mixed material directly into a mold.

It will be recognized from the foregoing that our method of forming all or part of the core material and then reducing the volume of the formed portion such as by preshrinkage and/or compression so that the same amount of material occupies less space, prior to completing the wall assembly, prevents failure of sandwich wall construction having temperature sensitive core ma terial when subjected to low temperatures.

Although the preceding discussion has been directed primarily to the application of the invention to double walled transport tanks and to the use of urethane foam, it will be clearly understood that the scope of the invention is not necessarily limited thereto, and applies to any cored sandwich construction, both annular and nonannular and to any temperature sensitive core material used therein which shrinks during formation thereof, after formation thereof when subjected to low temperatures, or both. The invention also applies to any sandwich construction in which the core material has inherent structural strength, engages both walls, and functions to support and maintain said walls a given or predetermined distance apart.

It will, of course, be understood that various changes may be made in the form, details, arrangement and proportions of the various parts without departing from the scope of our invention.

What is claimed is:

1. A method of forming a cored double walled structure of annular cross-section comprising spraying foamed core material on the major portion of the inner surface of the outer wall and curing, shrinking and self-bonding said sprayed foam on said outer wall to form a portion of the core, thereafter bringing said walls into fixed spaced apart opposed relationship whereby the sprayed foam and a void are disposed therebetween, and then pouring foamed core material into said void to substantially fill same, said poured material bonding itself to the inner wall and the sprayed foam whereby the inner and outer walls are integrally united by the foamed core material.

2. Annular wall structure comprising spaced apart inner and outer walls and a core of foam disposed therebetween, said core being self-bonded to both of said walls and uniting same and forming therewith an integral structure, said core comprising a portion which was foamed and applied to the major portion of one surface of one of said walls and at least partially cured and shrunk prior to the formation of said integral structure, and another portion which was foamed and cured simultaneously with the formation of said integral structure.

3. The annular wall structure of claim 2 wherein one of said core portions consist of a sprayed portion and the other of said core portions consisting of a poured portion.

4. The wall structure of claim 3, wherein said outer wall has sprayed foam bonded thereto.

5. The wall structure of claim 3, wherein said poured portion exerts pressure on and compresses the sprayed portion at normal temperatures. a

6. The wall structure of claim 3, wherein the sprayed portion constitutes the major portion of said core material.

7. The wall structure of claim 2, wherein one of said core portions includes laminae of sprayed foam self bonded to the major portion of one surface of each of said walls, and the other of said core portions including a lamina of poured foam interposed between and bonded to the sprayed foam laminae.

8. A method of forming wall structure having spaced apart walls and a core disposed therebetween and joined to both of said walls comprising forming a portion of said core and self-bonding it to the major portion of one surface of one of said walls, forming another portion of said core and self-bonding it to the major portion of one surface of the other of said walls, shrinking at least one of said core portions, thereafter bringing said walls with the core portions bonded thereto in fixed spaced apart relation to each other, and bonding said core portions together after said shrinkage has occurred whereby said walls are integrally joined by said core material.

9. A method of forming wall structure having spaced apart walls and "a core disposed therebetwe'enand joined to both of said walls comprising forming a portion of said core, self-bonding said core portion to the major portion of one surface of one of said walls, and reducing the volume of said core portion, thereafter bring said walls in fixed spaced apart opposed relationship wherein a void exists therebetween, and forming the remainder of said core material in said void and then self-bonding same to that which defines said void.

10. The method of claim 9 wherein the remainder of core material compresses said first formed portion of said core.

11. The method of claim 9, wherein said reduction in volume is accomplished by shrinking the core material.

12. The method of claim 9, wherein said wall structure is of annular cross-section.

13. The method of claim 12, wherein said reduction in volume is accomplished by shrinking the core material.

14. A method of forming a cored double walled structure comprising spraying foamed core material on the major portion of one surface of one of said walls and at least partially curing and shrinking said sprayed foam, thereafter bringing the foamed and unfoamed walls into fixed spaced apart opposed relationship whereby a void is formed between the spray foam and the unfoamed wall, then filling the void with poured foam and curing same, and bonding said cured poured foam to said sprayed portion and to the wall defining said void whereby said Walls are integrally joined together by said core material.

15. A method of forming a cored double walled structure comprising spraying and simultaneously self-bonding foamed core material individually on the major portion of one surface of each of said walls, at least partially curing and shrinking said sprayed foam, thereafter bringing said foamed Walls a given distance apart into fixed opposed relationship whereby a void exists between the foam atspaced apart opposed relationship with said foamed core portion and a void disposed therebetween, and then filling said void with the remainder of the foamed core material in such fashion that the pressure of forming the remainder of said core compresses the first foamed portion and whereby the walls are integrally joined by said core material.

17. The method of claim 16, wherein said wall structure is of annular cross-section.

References Cited by the Examiner UNITED STATES PATENTS 2,614,059 10/1952 Cooper 15679 2,855,021 10/1958 Hoppe. 2,858,580 11/ 1958 Thompson 204 X 2,869,336 1/1959 Smidl et al 204 X 2,887,732 5/1959 Kloote et al 204 2,928,456 3/1960 Potchen et a1 161161 X 2,957,832 10/1960 Gmitter et al. 2,976,577 3/ 1961 Gould. 3,003,199 10/1961 Talmey 204 3,013,922 12/1961 Fischer. 3,027,040 3/1962 Jodell et al. 264-54 X 3,030,256 4/ 1962 Rosenthal 161161 X 3,032,224 5/1962 Lou. 3,037,897 6/1962 Pelley 264-47 3,050,208 8/1962 Irvine 161190 3,072,582 1/1963 Frost. 3,080,267 3/ 1963 Schmaltz. 3,099,518 7/1963 Wetzler 15479 FOREIGN PATENTS 606,599 1960 Canada. 628,313 1961 Canada. 1,126,129 3/1962 Germany.

OTHER REFERENCES Polyurethanes by Dombrow (Reinhold Plastics Applications Series), pages 29 and 30, published in 1957 by Reinhold Publishing Corporation, New York, QD/3051/ A2D6.

FRANK L. ABBOTT, Primary Examiner.

JACOB L. NACKENOFF, Examiner.

J. E. MURTAGH, Assistant Examiner. 

2. ANNULAR WALL STRUCTURE COMPRISING SPACED APART INNER AND OUTER WALLS AND A CORE OF FOAM DISPOSED THEREBETWEEN, SAID CORE BEING SELF-BONDED TO BOTH OF SAID WALLS AND UNITING SAME AND FORMING THEREWITH AN INTEGRAL STRUCTURE, SAID CORE COMPRISING A PORTION WHICH WAS FOAMED AND APPLIED TO THE MAJOR PORTION OF ONE SURFACE OF ONE OF SAID WALLS AND AT LEAST PARTIALLY CURED AND SHRUNK PRIOR TO THE FORMATION OF SAID INTEGRAL STRUCTURE, AND ANOTHER PORTION WHICH WAS FOAMED AND CURED SIMULTANEOUSLY WITH THE FORMATION OF SAID INTEGRAL STRUCTURE. 